Long Range .243 Win Loads

I have a Rem 700 VLS in 243, using a Burris 3x9x40 balistic plex and have a pretty good load now using 85gr Sierra HPBT over 37gr of Varget. I'm getting an average of 3050fps on the chrony and averaging .5 MOA at 100 and under 2 in at 300.

So now I would like to begin trying to shoot 300+ but I think I need a bullet with a higher BC.

My rifle has a 1/9.125 twist. How long of bullet will it stabilize?

Which bullets and powder do you recomend? I currently have Varget, IMR4831, BLC2 and Benchmark.

Will this scope work with dailing in MOA or should I get a new one. I was thinking of looking to change the Bushnell 10x40 Tatical.